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By "hot-swap" I mean ensuring that the new battery is connected in parallel to the old battery while the old battery is removed and the new one is installed in its place. The idea is to ensure that the vehicle electricals/electronics are never without battery.

I've seen mechanics do this on a few Japanese vehicles (2008 Nissan Pathfinder, 2009 Mitsubishi Pajero).

I have also heard of one person lose central locking on his 2007 Nissan Patrol after failing to hot-swap his battery out during a replacement DIY.

Are these observations some kind of urban myth or are the vehicles designed like this from the factory? If yes, why? If not, why do mechanics (even at dealerships) observe this practice?

  • If nothing else, you might think a mechanic would do this as a courtesy, so that the customer doesn't have to reset all their radio stations. – Nate Eldredge Dec 30 '15 at 8:35
  • Probably has to do with electronics resetting - lost settings and volatile data. Some more electronics-dependent cars also like to throw fault codes in confusion after replacing the battery normally. – I have no idea what I'm doing Dec 30 '15 at 9:28
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    did this about 6 years ago on a vehicle I wasn't familiar with. I pulled the battery out and it erased all of the key fob mating memory. I bought a nice little tool that you plug a 9 volt battery into that keeps the ECM or CAN bus alive while you're doing stuf. They're called keep-alive memory modules or tools. – cloudnyn3 Dec 30 '15 at 12:20
  • CANbus is just a communication protocol, Its passive and therefor never considered "alive". Your tool provides 12V to the power line the comes from the ECM, Im guessing through the OBD? Or just into a cig lighter? They are handy but with it stepping the 9V battery to 13V (Yes 13) they dont last very long on vehicles with larger standby power drains like remote start systems or entertainment systems. – JpaytonWPD Dec 31 '15 at 3:02
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This is mostly a courtesy to customers to save driver settings that get lost when power is removed. Some cars can loose all their engine management setting requiring the car to "re-learn" how to run, resulting is poor fuel economy, poor performance, poor starting, and/or poor shifting.

As mentioned in the comments, revolving code key fobs work on a time clock based algorithm to make sure the codes are synced between the fob and the car. The codes change with every press and at set intervals over time. These cars often have a relearning mode that allows the car and fob to sync again after power loss.

Many devices are available to keep power available to the electronics while changing the battery. The most common method is to place a charger on the positive lead and a ground while the battery is removed. However this makes the installation more dangerous as sparks are more likely. Most newer vehicles make use of flash memory as its performance is on-par with older memory and it is not affected by the power loss. Some also make use of small builtin rechargeable batteries that maintain memory when power is lost.

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We use this type tool. It plugs into the ODBII port. There are many memorized settings that can be lost. The most troublesome are radio security codes and luxury cars with driver preference settings, such as seat and steering wheel position. The preference settings are the worst, since these cannot be duplicated without bothering the owner. Sit with an irate owner and a scan tool for 20 minutes while you reset the preference settings one time and you will pay anything for a memory saver.

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